Not reaching max revs - possible causes?

rwoofer

rwoofer

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My boat is less than a year old. I tried to give the engine full throttle today and only got 2600rpm and about 5.9 knots. The last time I tried full throttle earlier in the year I got 3000rpm (as designed) and 6.7 knots.

I scraped the prop only about 3 weeks ago, so I know it should be fairly clean. Bottom has a small amount of slime on it. Engine had 1st service about 4 weeks ago.

Any ideas what might be stopping me reaching max revs?
 
Plenty of things:

(1) Restriction in fuel supply, including clogged filter and blocked fuel tank vent;
(2) Fault in fuel lift pump;
(3) Restriction in air supply, including clogged filter;
(4) Air getting into fuel lines;
(5) Fault in injector pump, or timing incorrect;
(6) Blocked or faulty injector(s);
(7) Valves not seating correctly or timing not correct;
(8) Poor/unsuitable grade of diesel;
(9) Restricted or slipping throttle cable;
(10) Restriction in exhaust line;
(11) Blown cylinder head gasket;
(12) Damage or sudden wear to cylinders, pistons or piston rings;
(13) Overheating engine.

And that's just the engine. Problems with the transmission, rope or net round the prop could also be the cause. Then there is that old favourite, a defective rev counter, though that shouldn't affect your max speed.

The one thing it won't be is hull fouling. That may affect your speed, but not your revs.
 
Frist step, does the engine reach max revs under no load (out of gear). If so, then that eliminates a number of possibilities (throttle linkage, governor out of setting).

Second, can you turn the prop shaft easily when the engine is out of gear? If so, that eliminates such things as cutless or other bearing friction, ropes around the prop shaft etc.

Third, do you have some black smoke coming out at the back when at full throttle but only 2,600 revs? If so, the fuel is getting to the engine, but somewhere along the line there's too much load - low boat speed? fishing net wrapped around prop?

If no black smoke, then there's possibly not enough fuel getting to the injector pumps. Check filters first, then check for air leaks (groan, dismantling jobs . . .)

Narrow your search and it's much easier to find the cause . . .
 
HI not wishing to contradict other postings and I am not particularly knowledgeable about mechanical things but I do know that when my hull was dirty we lost about 1 knot and could not achieve full revs ( about 400 down I think) once cleaned and painted we got extra speed and revs
 
I agree with johnf the fouling is worse than you think
Its always this time of year that people complain about boat speed and in my limited experience its always fouling......
 
Thanks for the very useful replies. Having just had the engine serviced, I wasn't expecting the engine to be the problem, however....

Jim B, I like you diagnosis approach and think I will give it a go.

Having been prompted about the fouling and the amount it slows the boat, I think there is something in that. I was only dried out a couple of weeks ago and there was only slime on the hull - I just assumed slime didn't make too much of a difference, but maybe it does. Now I think about it, my sailing speeds have been low as well.
 
I would say almost certainly say dirty bottom (boat's that is!!) and fouled prop, even if you did scrape the prop a few weeks ago. I have a feathering prop and when it is dirty and hull is slimy I can only get 1800 - 2000 revs as against 3600 when it is nice an clean; I scraped my prop before lift out and managed to improve it from 1900 to 2200 but only got back to full revs after lift out and really good clean and anti-foul (hull and prop). So, don't panic it is almost certainly just fouling of boat and prop. /forums/images/graemlins/smirk.gif

Alan.
 
What is interesting about the fouling point is that in my 8 years of big boat ownership, I've never experienced a drop in max revs like this. My previous boat, a Moody 44 could keep max revs even when heavily fouled - it just went slower.

This makes me wonder if I'm slightly over-propped?
 
I scraped the prop of our boat on Saturday, as I could only manage 2500 rpm and 5 knots boat speed. At full normal revs, I can get 3000 (but don't usually run at that revs) and 7 knots. There wasn't a spare site for another barnacle to occupy on either side of the blades! It was like barnacle ghetto.

After the scrape and polish, and application of white spray grease (3 in 1 from B&Q) the revs would again reach 3000 and we had 7 knots again - even though I didn't touch the underwater hull, as mullet do such a good job of keeping it weed free.
 
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